The use of room temperature vulcanizing (RTV) silicone sealants for creating formed-in-place gaskets is well known in both original equipment manufacture and in engine repair and maintenance. A problem with conventional silicone sealants is their tendency to swell in the presence of oil, when the silicone is used to form a gasket. Improved resistance to swelling in the presence of oil is desirable.
Traditional means of improving oil resistance involve the use of comparatively high cross-link densities of the polymers. See, for instance, U.S. Pat. No. 4,257,932. The result is lower elongation properties and, hence reduced movement capabilities. Low modulus, high elongation, silicone rubbers are desirable as gaskets to accommodate joint movement due to vibration and thermal cycling. The lower modulus causes less stress to be exerted on the sealant-metal bond interface, decreasing the risk of failure.
U.S. Pat. No. 4,514,529, to Beers et al., discloses a low modulus RTV silicone sealant which is resistant to swelling, and has high elongation. The silicone elastomer is formed of silanol terminated diorganosiloxane base polymer, a tin catalyst, a ketoximosilane crosslinking agent, and low moisture calcium carbonate fillers. The compositions containing stearic acid treated CaCO.sub.3, according to Beers et al., have a modulus at 100% elongation of 110 psi (758 kPa) or less, with 90 psi (532 kPa) being preferred.
Beers, however, teaches that the moisture content of calcium carbonate must be limited to prevent curing of the sealant in the tubes. If small amounts of filler, 15% or less by weight in the sealant formulation, are used, the water level could range up to 0.4% by weight, based upon the total weight of the filler. For levels of filler over 15%, the water content is limited to a maximum of 0.2% by weight, with about 0.1% by weight, or less being preferred.
Sweet teaches ketoximosilane crosslinking agents for silicone RTV sealants in U.S. Pat. No. 3,189,576. Sweet also states that it is desirable that the filler be dry before admixing with the sealant composition. Sweet teaches that some water can be tolerated on the filler if an excess of ketoximosilane crosslinker is employed for reinforcing and non-reinforcing silicas, metallic oxides, and fibrous fillers such as asbestos or glass. No additional information is given as to the level of excess ketoximosilane to be employed. Sweet does not specify the use of calcium carbonates in oxime curing systems. White, et. al, in U.S. Pat. No. 4,395,526, teach silane scavengers that can be employed to remove hydroxy radicals such as the silanol radicals on silica filler used in acetoxy or alkoxy cure sealant. The silane scavenger has a generic formula, ##STR1## where R' is an aliphatic organic radical with from 1 to 8, inclusive, carbon atoms, R" is a monovalent organic radical from 1 to 13, inclusive, carbon atoms, and X is a hydrolyzable leaving group. Oximato groups are included as possible hydrolyzable leaving groups, but trioximo or tetraoximo silanes are not disclosed. The White, et. al patent also does not address scavengers in an oxime cure system, using calcium carbonate fillers.
Dziark, in patent application WO 93/19130, discloses an oil resistant silicone made from a combination of a silanol terminated diorganosiloxane polymer, a trimethyl-endblocked polydiorganosiloxane fluid, a silica filler, a calcium carbonate filler, gamma-aminopropyl triethoxysilane, a tin catalyst, and methyl or vinyltris(methylethylketoximo)silane crosslinking agent. The calcium carbonate is shown to improve the oil resistance, while the gamma-aminopropyl triethoxysilane is shown to improve adhesion. Dziark's compositions require the use of a silica filler.